Dual band, low sidelobe, high efficiency mirror antenna

ABSTRACT

A dual band mirror antenna employing a continuous mirror without a hole.  e-by-side feeds for each band and an electromagnetic lens, located behind a rotatable twist reflector (the mirror), collimate beams toward a pair of polarized reflectors located near the twist reflector and tilted to aim the reflected energy toward the twist reflector. One polarized reflector is reflective at the higher radiofrequency band and transparent at the lower radiofrequency band. The other polarized reflector is reflective at both bands. Energy directed back toward the polarized reflectors from the twist reflector passes through the polarized reflectors to free space.

BACKGROUND OF THE INVENTION

The present invention relates generally to antennas for radiofrequencyenergy, and more particularly to antennas required to produceelectromagnetic beams over wide angles of coverage volume.

U.S. Pat. No. 4,070,678 issued to Richard L. Smedes on Apr. 2, 1976discloses a two-axis mirror antenna. This antenna has a fixed axial feedwhich illuminates a fixed wire grid parabola supported by a radome. Thefeed polarization is parallel to the grid wires of the parabola. Theparabola forms the energy into a beam aimed back toward a mirrorsurrounding the feed. The mirror is a "half-wave plate" which rotatespolarization 90° and reflects the beam into space through a sphericallens which collimates the beam. This energy, being polarized orthogonalto the grid wires forming the parabola, flows through the parabola withnegligible attenuation. The echo from targets reverses the procedure tobe focused onto the feed. The beam is moved by tilting the mirror,giving a beam shift of approximately twice the mirror tilt angle.

The mirror antenna is a very effective device for rapid large angle beamscanning, but the hole in the mirror for the feed limits sidelobeperformance and causes some loss.

The prior art antenna is only capable of providing a beam of energy atwavelengths in a single radiofrequency band and there is no suggestionof modifying it for dual-band operation.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to obtain very low sidelobeswith a two-axis mirror antenna operating at two radiofrequency bandssimultaneously.

Another object is to maximize the efficiency of a two-axis mirrorantenna operating at two radiofrequency bands simultaneously.

These and other objects of the invention are achieved by a mirrorantenna which includes first and second feed horns symmetricallydisposed about a longitudinal axis for forming linearly-polarizeddivergent beams of energy at wavelengths in a higher and lowerradiofrequency band respectively; an electromagnetic lens forsimultaneously refracting and collimating the divergent beams; a firstfixed polarized reflector for reflecting one beam while transmitting theother beam; a second fixed polarized reflector for reflecting thetransmitted beam along the same path as travelled by the reflected beam;and a rotatably mounted twist reflector having a continuous reflectingsurface for changing the direction of the reflected beams in accordancewith the position of the twist reflector and for twisting theirpolarization by substantially 90° so that if the beams are directed backtoward the polarized reflectors, the beams pass through the polarizedreflectors to free space. Radiofrequency absorbent material may be addedbehind the lens to eliminate significant spillover lobes.

The use of a continuous reflecting surface in the mirror antenna designeliminates the loss of energy which occured in the prior art mirrorantenna because of energy falling on the hole. The design allows a verylow sidelobe device to collimate the beam with a minimum degradation ofthe pattern formed by the device. The two feed horns can be optimizedindividually and independently for optimum tapers, to further reducesidelobes, and to maximize monopulse performance when the feed horns areused as monopulse feeds.

Additional advantages and features will become apparent as the subjectinvention becomes better understood by reference to the followingdetailed description when considered in conjunction with theaccompanying drawing wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of the invention.

FIG. 2 shows a plan view of reflector 19.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the Figure, the dual-band mirror antenna 10 includes ahigher-radiofrequency-band feed horn 11 and a lower-radiofrequency-bandfeed horn 13 symmetrically disposed about a longitudinal axis 15; anelectromagnetic lens 17 disposed along the longitudinal axis in the pathof beams from the feed horns; a polarized reflector 19, such as adichroic grating, which is reflective at the higher radiofrequency bandand transparent at the lower radiofrequency band, disposed in fixedspatial relationship to the electromagnetic lens; a polarized reflector21, such as a grating, disposed behind the reflector 19 in fixed spatialrelationship to the electromagnetic lens; and a twist reflector 23 whichhas a continuous reflecting surface and is rotatably mounted in thereflecting path of the reflectors 19 and 21.

Suitable twist reflectors 23 are described, for example, in the article"A Broad-Band Twist Reflector" by Lars G. Josefsson in IEEE Trans. onAntennas and Propagation (July 1971) pp. 552-554, whose disclosure isherewith incorporated by reference. The twist reflector 23 is mounted ona positioner 25 for rotation about two mutually perpendicular axis, suchaxes being perpendicular to the paper, and in the plane of the paper,respectively. A suitable positioner 25 is described, for example, inU.S. Pat. No. 3,374,977 issued to George Moy, Jr. on Mar. 26, 1968,herewith incorporated by reference.

As shown in FIG. 1, radiofrequency absorbent material 27 can be disposedaround the back of the electromagnetic lens 17.

Referring to FIG. 2, there is shown a plan view of a portion ofreflector 19 which includes a two-dimensional array of conductingplates, whose dimensions and spacings determine the frequency bandreflected by the reflector as is known in the art. Obviously, somesupporting structure (not shown) should be provided to support theplate.

Details of the construction of this reflector and the theory of itsperformance may be found in the publication: Radio Science, Vol. 2 (NewSeries), No. 11, November 1967, page 1347-1359.

In operation, the feed horns 11 and 13 are connected to a transmitter(not shown). The feed horn 11 forms a linearly-polarized divergent beam29 of energy at wavelengths in a higher radiofrequency band; and thefeed horn 13 forms a linearly-polarized divergent beam 31 of energy atwavelengths in a lower radiofrequency band. The electromagnetic lens 17simultaneously refracts and collimates the radiofrequency energy in eachbeam to produce a linearly-polarized collimated beam 33 of energy atwavelengths in the higher radiofrequency band and a linearly-polarizedcollimated beam 35 of energy at wavelengths in the lower radiofrequencyband. The beams 33 and 35 are respectively offset one from the other.The linearly-polarized collimated beams illuminate the polarizedreflector 19, the polarization being perpendicular to the plane of thepaper, say "vertical". The polarized reflector 19 reflects thelinearly-polarized collimated beam 33 of energy at wavelengths in thehigher radiofrequency band onto the continuous surface of the twistreflector 23, while transmitting the linearly-polarized collimated beam35 of energy at wavelengths in the lower radiofrequency band to thepolarized reflector 21 which reflects the transmitted beam 35 onto thetwist reflector 23 along the same path as travelled by beam 33. Thetwist reflector 23 changes the direction of the linearly-polarizedcollimated beams of radiofrequency energy in accordance with theposition of the twist reflector, and twists the polarization of theradiofrequency energy in the collimated beams by 90 degrees. That is,the polarization of the radition reflected from the twist reflector 23is made horizontal, or in the plane of the paper (the terms "vertical"and "horizontal" are used for convenience, not with any limiting force).Such radiation will, if directed back towards the polarized reflectors19 and 21 as shown in FIG. 1, pass through to free space. By rotatingthe twist reflector 23 about mutually perpendicular axes, the beams 37and 39 can be aimed into space over a large coverage volume.

The radiofrequency absorbent material 27 eliminates spillover sidelobeswhose position in space is fixed, even though the main beams are movedby the twist reflector 23.

The surface of the twist reflector 23 is continuous, unlike that of themirror for the feed in the antenna assembly shown in the above-citedU.S. Pat. No. 4,070,678 wherein energy is lost to the hole in themirror. Furthermore, the absence of a mirror hole permits a reduction insidelobe level.

It is obvious that many modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A dual band, low sidelobe, high efficiencymirror antenna comprising:a first feed horn for forming alinearly-polarized divergent beam of energy at wavelengths in a higherradiofrequency band; a second feed horn for forming a linearly-polarizeddivergent beam of energy at wavelengths in a lower radiofrequency band;the first and second feed horns being symmetrically disposed about alongitudinal axis; an electromagnetic lens disposed along thelongitudinal axis in the path of the linearly-polarized divergent beamsfrom the first and second feed horns for simultaneously refracting andcollimating the radiofrequency energy in each beam to produce alinearly-polarized collimated beam of energy at wavelengths in thehigher radiofrequency band and a linearly polarized collimated beam ofenergy at wavelengths in the lower radiofrequency band, the collimatedbeams being respectively offset one from the other; a first polarizedreflector, reflective at one radiofrequency band and transparent at theother radiofrequency band, disposed in fixed spatial relationship to theelectromagnetic lens for reflecting the linearly-polarized collimatedbeam of energy at wavelengths in the one radiofrequency band whiletransmitting the linearly polarized collimated beam of energy atwavelengths in the other radiofrequency band; a second polarizedreflector disposed behind the first reflector in fixed spatialrelationship to the electromagnetic lens for reflecting the transmittedlinearly-polarized collimated beam of energy at wavelengths in the otherradiofrequency band along the same path as travelled by the reflectedlinearly-polarized collimated beam of energy at wavelengths in the oneradiofrequency band; and a twist reflector having a continuousreflecting surface and rotatably mounted in the path of the reflectedlinearly-polarized collimated beams for changing the direction of thelinearly-polarized collimated beams of radiofrequency energy inaccordance with the position of the twist reflector and for twisting thepolarization of the radiofrequency energy in the collimated beams bysubstantially 90 degrees so that if the beams are directed back towardthe first and second polarized reflectors, the beams pass through thefirst and second polarized reflectors to free space.
 2. The mirrorantenna recited in claim 1 including:radiofrequency-absorbent materialdisposed at the electromagnetic lens to eliminate spillover sidelobes.3. A high efficiency, low sidelobe method of directing dual bandcollimated beams of radiofrequency energy comprising the stepsof:forming a linearly-polarized divergent beam of energy at wavelengthsin a higher radiofrequency band; forming a linearly-polarized divergentbeam of energy at wavelengths in a lower radiofrequency band;simultaneously refracting and collimating the radiofrequency energy ineach beam to produce a linearly-polarized collimated beam of energy atwavelengths in the higher radiofrequency band and a linearly-polarizedbeam of energy at wavelengths in the lower frequency band; reflectingalong the same path the linearly-polarized collimated beam of energy atwavelengths in the higher radiofrequency band and the linearly-polarizedcollimated beam of energy at wavelengths in the lower radiofrequencyband; changing the direction of the linearly-polarized collimated beamsof radiofrequency energy; and twisting the polarization of theradiofrequency energy in the collimated beams by substantially 90degrees.